1. Field of the Invention
The invention relates to a single-electrode lead, in particular for implantable cardioverter defibrillators (ICDs) comprising a tubular, flexible lead body; a ventricular tip electrode; a ventricular, in particular helical shock electrode; an atrial, in particular helical shock electrode; and electric lines guided in the lead body to the individual electrodes.
2. Background Art
As regards the background of the invention, implantable defibrillators are customarily equipped with single-electrode leads, which are inserted via the vena cava and the right atrium into the right cardiac ventricle. Defibrillators serve among other things for the treatment of tachycardia conditions in which the heart, beating at a pathologically high frequency, is reset to its normal condition by the delivery of a shock of an electric voltage with amplitudes of some hundreds of volts.
So as to be able to detect the condition of the heart, implantable cardioverter defibrillators have input channels for the signals measured atrially and ventricularly in the heart and treated and amplified electrically in the input channels. These input channels are also utilized for the customary heart pacemaker functions that an ICD normally has. For example, a VDD heart pacemaker comprises the steps of ventricular (=V) stimulation, and atrial and ventricular (D=double) detection of the cardiac signals, the ventricular stimulation taking place only upon demand (=D), i.e. when the heart shows no ventricular self-stimulated action.
Problems are posed by the detection of the electric signal, to be measured, of atrial stimulation, which is designated as P wave in electrocardiography. This signal is comparatively weak and, therefore, hard to detect. Moreover, the signal quality to be measured depends decisively on the properties of the electrodes positioned in the atrium. In the case of single-electrode leads of heart pacemakers and ICDs, either annular or helical electrodes are placed in the atrium, floating in the blood stream without contacting the cardiac wall.
Annular electrodes of heart pacemakers only have few millimeters of length, therefore offering acceptable sensing properties. Shock electrodes of defibrillators must be of large surface design in order for too high local power densities to be prevented from originating in spite of the high electric powers which are emitted upon delivery of a shock. These power densities might damage the myocardium.
It is known that atrial signals (P wave) can be detected by the ventricular and atrial shock electrode. However, a problem resides in that the shock electrodes conventionally have a length exceeding the electric wave front which migrates along the myocardium during the stimulation of the heart. This electric wave front leads to the signal to be detected. In the case of an elongated electrode, the positive and negative amplitudes of the wave front integrate approximately to zero and the signals thus measurable are restricted primarily to weak initial and final amplitudes. These are produced when the wave front reaches or leaves the helical shock electrode.
So as to improve the detection properties, it is conceivable, similarly to the ventricular tip electrode or annular electrode (provided the latter is available) to mount a corresponding additional electrode of comparatively small dimensions in the atrial area of the electrode lead. However, this would result in at least one further electric line being added to the existing at least three lines in the lead body. More and more lines must be provided for further electrodes, which would result in a thicker and, above all, more rigid electrode lead. However, this is not desirable with a view to simplest possible implantability and tolerance of the lead.